1. Field of the Invention
This invention relates to electret microphones, and more particularly to miniature electret microphones and methods for manufacturing miniature electret microphones.
2. Description of Related Art
An electret is a dielectric that produces a permanent external electric field which results from permanent ordering of molecular dipoles or from stable uncompensated surface or space charge. Electrets have been the subject of study for their charge storage characteristics as well as for their application in a wide variety of devices such as acoustic transducers (including, for example, hearing aids), electrographic devices, and photocopy machines.
A number of electret microphone designs exist. However, small, high quality electret microphones tend to be quite expensive. Therefore, a need exists for small, high quality, inexpensive electrets, particularly electret microphones. The present invention meets these needs.
The present invention uses micro-machining technology to fabricate a small, inexpensive, high quality electret on a support surface, and further uses micro-machining technology to fabricate a small, inexpensive, high quality, self-powered electret sound transducer, preferably in the form of a microphone. Each microphone is manufactured as a two-piece unit, comprising a microphone membrane unit and a microphone back plate, at least one of which includes an electret formed by micro-machining technology. When juxtaposed, the two units form a highly reliable, inexpensive microphone that can produce a signal without the need for external biasing, thereby reducing system volume and complexity.
In the preferred embodiment, the electret material used is a thin film of spin-on polytetrafluoroethylene (PTFE). An electron gun preferably is used for charge implantation. The electret has a saturated charged density in the range of about 2xc3x9710xe2x88x925 C/m2 to about 8xc3x9710xe2x88x924 C/m2. Thermal annealing is used to stabilize the implanted charge.
Two prototype micro-machined electret microphones have been fabricated and tested. An open circuit sensitivity of about 0.5 mV/Pa has been achieved for a hybrid microphone package.